By using selective amplifier circuits embodied as tuners, various broadcast programs, such as radio or television programs, or any signal in general can be received on a selected one of various frequencies, amplified and then reproduced. Tuning circuits are necessary for selecting the desired receiving frequency, which makes it possible to receive a particular signal that is being transmitted on this frequency. In this context, an oscillator circuit establishes an oscillator frequency that is shifted by a fixed intermediate frequency relative to the desired receiving frequency, and then provides this oscillator frequency to a mixer. Input, drain, and/or intermediate circuits, which serve as a frequency filter for filtering at the receiving frequency are connected in the circuit before the mixer. The reception signal is prefiltered through the input, drain and/or intermediate circuits, and then the prefiltered signal is also provided to the mixer. The fixed intermediate frequency is formed from the oscillator frequency and the prefiltered reception signal.
In this context, the tuning circuit serves to ensure that the input circuit, drain circuit, and/or intermediate circuits are tuned to the desired receiving frequency, and that the oscillator circuit is tuned to the corresponding oscillator frequency that is frequency shifted by the intermediate frequency (IF) relative to the receiving frequency. When all of the circuits are tuned to the respective intended nominal frequencies, this condition is designated as aligned, balanced or synchronous operation, with which the optimum reception is ensured.
The prior art methods of achieving such calibrating and tuning of a multistage selective amplifier have suffered various difficulties and disadvantages. Generally according to the known methods, all the tuning voltages of all the tuning circuits of the amplifier, which respectively set these circuits to the desired frequency, had to be stored in a microprocessor, or had to be determined through a complex and time consuming iterative process. The known tuning circuits for carrying out such known methods had to be able to store a large quantity of data relating to the tuning voltages, or had to carry out a lengthy iterative process, whereby the desired value could only be slowly approximated in an iterative stepwise manner.
Published European Patent Application 0,044,237 (Shepherd et al.) discloses a method of tuning an electronic circuit, and particularly a television receiver, using a plurality of variable elements and carrying out a regulation of the oscillator frequency by means of a phase-locked loop (PLL). Particularly, this reference refers to tuning a filter or oscillator using capacitance diodes. The known calibrating apparatus for carrying out the known method comprises a programmable read-only memory (PROM) that is already permanently programmed at the time of manufacturing the circuit, to store the characteristic curve function by means of which the respective desired values can be calculated. During operation of the circuit, the stored data are used to generate control signals for carrying out the electronic tuning of the circuit.
However, in this known arrangement, it is disadvantageous that the characteristic curve function is already stored in the memory at the time of manufacturing the circuit, because any possible changes or differences that might arise over time cannot be taken into account. In other words, any purposeful change in the circuitry, or in the desired tuning characteristics, or any other variation or deviation over time, requires a reprogramming of the memory, or a newly programmed memory. Any device specific deviations cannot be taken into account with this method or apparatus. Moreover, it is important in this known method to store as many points of the characteristic curve as possible in the memory, because the accuracy of the method is dependent on the number of the stored characteristic curved points. As a further disadvantage, it must be considered that this known method necessarily involves a highly active data transfer traffic when changing frequencies. Namely, each change in frequency requires new data values to be written or transferred from the memory into the phase-locked loop.
In known electronically tunable selective amplifiers, and particularly tuners, voltage controlled reactance elements such as capacitance diodes are used for tuning the pre-circuit or input circuit, intermediate circuit, and oscillator circuit. In these respective circuits, separate and independently variable tuning voltages are provided to the respective capacitance diodes that are used for the tuning. Depending on the selection of a new channel or frequency, the tuner is automatically calibrated or aligned to this new frequency by means of a microprocessor in connection with the independently adjustable tuning voltages. In order to achieve this, the microprocessor first disconnects the antenna from the input of the input circuit, and instead couples thereto a phase-locked loop controlled oscillator that is oscillating at the respective present frequency of the respective selected channel. The calibration or aligning of the individual circuits is carried out successively for each circuit by varying the respective tuning voltage that is allocated to the respective circuit. A detector connected to the output of the last circuit serves to signal when the maximum of the bandpass curve has been achieved, whereupon the calibration of the individual circuits is ended. After the end of the overall calibrating process, the antenna is once again connected to the input circuit instead of the oscillator. The tuning voltages that have been determined in this manner are then digitally stored until a new channel or frequency is selected, whereupon the calibrating process will begin anew.
In contrast to the above described known method, European Patent 0,147,518 (Heucke-Gareis et al.) discloses a method of carrying out the calibrating process using an external connectable, automatically or manually operating calibrating device. This calibrating device comprises an external intermediate memory, which intermediately stores the tuning steps or tuning parameters that have been determined according to an iterative tuning process, and then transmits optimized tuning values into a permanent or non-volatile internal memory allocated to an amplifier or tuner. In the later selection of a different channel or frequency, these stored values are then called-up from the memory and the individual tuning voltages are correspondingly adjusted. Because this internal memory must be capable of storing considerable quantities of data, the method also provides for calibrating only each n.sup.th channel and then interpolating the necessary tuning values for the channels lying therebetween.
One major disadvantage of the above described known method is the high memory space requirement, and a second major disadvantage is the data traffic that is necessary for each change of frequency or channel. The necessity of carrying out such a data transfer substantially disadvantages a rapid searching or seeking function or a frequency jump function for tuning to a new frequency. Such functions are not very important in present television tuners, but are extremely important in present day car radios. Namely, these functions make it possible to jump to a second alternative transmitting frequency of the same broadcast program in a rapid and preferably inaudible manner in the event of diminishing reception quality on the originally selected frequency, or to carry out a rapid seeking or searching of successive transmitting frequencies for briefly monitoring the programs present on these frequencies. Such a rapid frequency jumping function is also necessary when laying out a table of the programs to be received on the various frequencies.
German Patent Laying-Open Document 2,524,171 (Rinderle) discloses a tuning circuit that provides the necessary tuning voltage by means of a tuning potentiometer, and that includes further trimming potentiometers between the individual tuning circuits for achieving the desired aligned operation. The use of such tuning and trimming potentiometers is considered to be inexact and subject to operating errors or deviations, and does not achieve rapid and exact tuning to a selected frequency.
It is further known to use a phase-locked loop for regulating the oscillator frequency, instead of using a tuning potentiometer. This known arrangement has the advantage that a processor simply provides the desired oscillator frequency to the phase-locked loop, and the aligned operation is then necessarily defined by the setting of the calibrating potentiometer.